The present invention relates to a workpiece engaging or aligning apparatus for use in a grinding machine, and more particularly to a workpiece engaging or aligning apparatus for bringing a workpiece such as a gear into optimum engagement or mesh with a grinding wheel when the workpiece is to be ground by the rotating grinding wheel in a grinding machine.
There have been used in the art grinding machines for grinding workpieces such as gears with a grinding wheel having helical teeth on an outer peripheral surface thereof which are held in mesh with gear teeth. Before grinding the gear teeth, the gear is displaced toward the grinding wheel by a feed motor or the like, and then gear teeth are brought into mesh with the helical teeth on the grinding wheel. Where the gear to be ground is of an eccentric shape or has irregular slots or grooves between the gear teeth, the gear teeth surfaces will be excessively ground. Generally, the grinding allowance accepted in the art to be suitable for gear teeth is in the range of from 0.05 mm to 0.10 mm. It has also been confirmed in the art that for mass-produced gears, the eccentricity of the diameter of a root circle of the gears which serves as a reference when grinding the gear teeth surfaces should be in a tolerance range below about 0.5 mm for a good grinding efficiency. Assuming that the gear root circle eccentricity and irregular gear grooves are expressed wholly as an equivalent eccentricity .epsilon., each gear tooth surface is required to have a grinding allowance of 2 .epsilon.. As an example, a gear having an equivalent eccentricy .epsilon. of 0.05 due to an eccentric gear shape and uneven gear grooves has an overall eccentricity 2 .notident. of 0.1, which is comparable to a normal grinding allowance.
The equivalent eccentricity will be described in greater detail by way of example.
When grinding a gear, it is customary practice to bring a grinding wheel and the gear which have mutually perpendicular rotational axes into mesh with each other, and then disconnect a cluch on a shaft for rotating the gear for matching the phases of the grinding wheel and the gear. The gear is now allowed to rotate with the rotation of the grinding wheel. When a feed motor is energized, the gear is advanced into mesh with the grinding wheel until no backlash is produced between the gear and the grinding wheel, whereupon the clutch is connected again to rotate the grinding wheel and the gear in synchronism with each other.
FIGS. 1 and 2 of the accompanying drawings illustrate, in a modified fashion, conditions in which the center of a gear or workpiece W does not coincide with the center of rotation of the workpiece W. More specifically, in FIG. 1(a), the center of gravity GC of the gear W is displaced off the center of rotation WC of the gear W in a direction toward a point where the gear W meshes with the grinding wheel T. When the gear W is turned 90.degree. from the position of FIG. 1(a), the gear W assumes the position shown in FIG. 1(b). In the position of FIG. 1(b), a thickness of the gear W which corresponds to an eccentricity of the gear W is ground off excessively by cutting edges of the grinding wheel T. As the gear W is further turned 180.degree. from the position of FIG. 1(b), another thickness of the gear W equivalent to the eccentricity thereof is ground off by the grinding wheel T.
In the condition of FIG. 2(a), the center of gravity GC of the gear W is displaced off the center of rotation WC of the gear W in a direction that is angularly spaced 90.degree. from a point of meshing contact between the gear W and the grinding wheel T. When the gear W is angularly moved 180.degree. from the position of FIG. 2(a) to the position of FIG. 2(b), a thickness corresponding to the shaded area (FIG. 3) or 2 will be ground off by the grinding wheel T.
Therefore, if the center of gravity GC of the gear W is displaced off the center of rotation WC thereof in a direction angularly spaced 90.degree. from the point of meshing contact with the grinding wheel T, then the grining wheel T requires the gear W to have a grinding allowance which is twice that with respect to the meshing condition shown in FIG. 1. Stated otherwise, where the same grinding allowance as that shown in FIG. 2 is to be ground off and if the gear W and the grinding wheel T are kept in mesh with each other as illustrated in FIG. 1, then the gear W is permitted to be eccentric to the extent which is twice that shown in FIG. 1. Therefore, the grinding operation can be effected more easily in the condition of FIG. 1 than in the condition of FIG. 2.